A Revisit of Parametrization of Downward Longwave Radiation In
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1 A revisit of parametrization of downward longwave radiation in 2 summer over the Tibetan Plateau based on high temporal resolution 3 measurements 4 Mengqi Liua,c, Xiangdong Zhengd, Jinqiang Zhanga,b,c and Xiangao Xiaa,b,c 5 a LAGEO, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 6 100029, China 7 b Collaborative Innovation Center on Forecast and Evaluation of Meteorological 8 Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, 9 China 10 c College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, 11 Beijing, 100049, China 12 d Chinese Academy of Meteorological Sciences, Chinese Meteorological Bureau, 13 Beijing, 100081, China 14 15 1 16 Abstract 17 The Tibetan Plateau (TP) is one of research hot spots in the climate change research 18 due to its unique geographical location and high altitude. Downward longwave 19 radiation (DLR), as a key component in the surface energy budget, is of practical 20 implications for radiation budget and climate change. A couple of attempts have been 21 made to parametrize DLR over the TP based on hourly or daily measurements and crude 22 clear sky discrimination methods. This study uses 1-minute shortwave and longwave 23 radiation measurements at three stations over TP to parameterize DLR during summer 24 months. Three independent methods are used to discriminate clear sky from clouds 25 based on 1-minute radiation and Lidar measurements. This guarantees strict selection 26 of clear sky samples that is fundamental for the parameterization of clear-sky DLR. 27 Eleven clear-sky and four cloudy DLR parameterizations are examined and locally 28 calibrated. Comparing to previous studies, DLR parameterizations here are shown be 29 characterized by smaller root mean square error (RMSE) and higher coefficient of 30 determination (R2). Clear-sky DLR can be estimated from the best parametrization with 31 RMSE of 3.8 W⸱m-2 and R2 > 0.98. Systematic overestimation of clear-sky DLR by the 32 locally calibrated parametrization in one previous study is found to be approximately 33 25 W⸱m-2 (10%), which is very likely due to potential residual cloud contamination on 34 previous clear-sky DLR parametrization. Cloud-base height under overcast conditions 35 is shown to play an important role in cloudy DLR parameterization, which is considered 36 in the locally calibrated parameterization over the TP for the first time. Further studies 37 on DLR parameterization during nighttime and in seasons except summer are required 38 for our better understanding of the role of DLR in climate change. 39 40 2 41 1 Introduction 42 The downward longwave radiation (DLR) at the Earth’s surface is the largest 43 component of the surface energy budget, being nearly double the downward shortwave 44 radiation (DSR) (Kiehl and Trenberth, 1997). DLR has shown a remarkable increase 45 during the process of global warming (Stephens et al., 2012). This is closely related to 46 the fact that both a warming and moistening of the atmosphere (especially at the lower 47 atmosphere associated with the water vapor feedback) positively contribute to this 48 change. Understanding of complex spatiotemporal variation of DLR and its implication 49 is necessary for improving weather prediction, climate simulation as well as water 50 cycling modeling. Unfortunately, errors in DLR are considered substantially larger than 51 errors in any of the other components of surface energy balance, which is most likely 52 related to the lack of DLR measurements with high quality (Stephens et al., 2012). 53 The 2-sigma uncertainty of DLR measurement by using a well-calibrated and 54 maintained pyrgeometer is estimated to be 2.5% or 4 W⸱m-2 (Stoffel, 2005). However, 55 global-wide surface observations are very limited, especially in some remote regions. 56 On the other hand, it has been known for almost one century that clear-sky DLR is 57 determined by the bulk emissivity and effective temperature of the overlying 58 atmosphere (Ångström, 1918). Since these two quantities are not easily observed for a 59 vertical column of the atmosphere, clear-sky DLR is widely parameterized as a function 60 of surface air temperature and water vapor density, assuming that the clear sky radiates 61 toward the surface like a grey body at screen-level temperature. Dozens of 62 parameterization formulas of DLR have been developed in which clear-sky effective 63 emissivity (εc) is a function of the screen-level temperature (T) and water vapor pressure 64 (e) (T and e have the same meaning and unit in following equations if not specified), or 65 simply in the localized coefficients with given functions. Two formulas, i.e., an 66 exponential function (Idso, 1981) and a power law function (Brunt, 1932; Swinbank, 67 1963), have been widely used to depict the relationship of εc to T and e. The coefficients 68 of these functions are derived by a regression analysis of collocated measurements of 69 T, e and DLR. Most of these proposed parameterizations are empirical in nature and 70 only specific for definite atmospheric condition. An exception is that Brutsaert (1975) 3 71 developed a model based on the analytic solution of the Schwarzchild’s equation for a 72 standard atmospheric lapse rates of T and e. Prata (1996) found that the precipitable 73 water content (w) was much better to represent the effective emissivity of the 74 atmosphere than e, which was loosely based on radiative transfer simulations. Dilley 75 and O’Brien (1998) adopted this scheme but tuned empirically their parameterization 76 using an accurate radiative transfer model. Since DLR is to some extent impacted by 77 water vapor and temperature profile (especially in case of existence of an inversion 78 layer) and diurnal variation of T, a new model with two more coefficients considering 79 these effects was developed (Dupont et al., 2008). 80 In the presence of clouds, total effective emissivity of the sky is remarkably 81 modulated by clouds. The existing clear-sky parameterization should be modified 82 according to the cloud fraction (CF) and other cloud parameters such as cloud base 83 height (CBH). CF is generally used to represent a fairly simple cloud modification 84 under cloudy conditions. Dozens of equations with cloudiness correction have been 85 developed and evaluated by DLR measurements across the world (Crawford and 86 Duchon, 1999; Niemelä et al., 2001). CF can be obtained by trained human observers 87 (Iziomon et al., 2003) or derived from DSR (Crawford and Duchon, 1999) and DLR 88 measurements (Dürr and Philipona, 2004). High temporal resolution of DSR or DLR 89 measurements (for example, 1-minute) can also provide cloud type information 90 (Duchon and O’Malley, 1999), and thereby allow to consider potential effects of cloud 91 types on DLR (Orsini et al., 2002). 92 With an average altitude exceeding 4 km above the sea level (ASL), the Tibetan 93 Plateau (TP) exerts a huge influence on regional and global climate through mechanical 94 and thermal forcing because of its highest and most extensive highland in the world 95 (Duan and Wu, 2006). TP, compared to other high altitude regions and the poles, has 96 been relatively more sensitive to climate change. The most rapid warming rate over the 97 TP occurred in the latter half of the 20th century likely associated with relatively large 98 increase in DLR. Duan and Wu (2006) indicated that increase in low level nocturnal 99 cloud amount and thereby DLR could partly explain the increase in the minimum 100 temperature, despite decrease in total cloud amount during the same period. By using 4 101 observed sensitivity of DLR to change in specific humidity for the Alps, Rangwala et 102 al. (2009) suggested that increase in water vapor appeared to be partly responsible for 103 the large warming over the TP. Since the coefficients of certain empirical 104 parameterizations and their performances showed spatiotemporal variations, 105 establishment of localized DLR parameterizations over the TP is of high significance. 106 Further studies on DLR, including its spatiotemporal variability, its parameterization as 107 well as its sensitivity to changes in atmospheric variables, would be expected to 108 improve our understanding of climate change over the TP (Wang and Dickinson, 2013). 109 DLR measurements from high quality radiometer with high temporal resolution 110 over the TP are quite scarce. To the best of our knowledge, there are very few 111 publications on DLR and its parameterization over the TP. Wang and Liang (2009) 112 evaluated clear-sky DLR parameterizations of Brunt (1932) and Brutsaert (1975) at 36 113 globally distributed sites, in which DLR data at two TP stations were used. Yang et al. 114 (2012) used hourly DLR data at 6 stations to study major characteristics of DLR and to 115 assess the all-sky parameterization of Crawford and Duchon (1999). Zhu et al. (2017) 116 evaluated 13 clear-sky and 10 all-sky DLR models based on hourly DLR measurements 117 at 5 automatic meteorological stations. The Kipp & Zonen CNR1 is composed of CM3 118 pyranometer and CG3 pyrgeometer that are used to measure DLR and DSR, 119 respectively. The CG3 is the second class radiometer according to the International 120 Organization for Standardization (ISO) classification. The root mean square error of 121 hourly DLR is less than 5 W⸱m-2 after field recalibration and window heating correction 122 (Michel et al., 2008). Note that human observations of cloud every 3-6 hours or hourly 123 DLR and DSR data are respectively used to determine clear sky and cloud cover in 124 these previous studies. 125 In order to further our understanding of DLR and DSR over the TP, measurements 126 of 1-minute DSR and DLR at 3 stations over the TP using state-of-the-art instruments 127 have been performed in summer months since 2011.